It is becoming apparent that in the future, stationary applications, for example, wind turbines, as well as vehicles, for example, electric and hybrid vehicles, will increasingly use modularly configured electric energy storage devices, in particular batteries or battery devices, as voltages sources, on which very high demands will be made with respect to reliability. The reason for these high demands is that a failure of an energy storage module of the device, for example, a battery cell of the battery device, may result in a failure of the overall system. For example, in an electric vehicle, a failure of the traction battery thus results in a stranded vehicle. The failure may even result in safety-related problems. Thus, in wind turbines, batteries are used as energy stores in order to protect the wind turbine from unallowable operating conditions during heavy wind via a rotor blade adjustment. If a failure occurs, the wind turbine may enter such an unallowable operating state under some circumstances.
The energy storage modules are often connected to an energy storage device in the intermediate circuit (DC link circuit) of an inverter system or a converter system. This intermediate circuit is provided with a capacitive component, the intermediate-circuit capacitor, for keeping the DC voltage constant and for suppressing voltage peaks. In principle, the capacitive element must be pre-charged for this purpose. The energy storage modules of the electric energy storage device are connected in parallel with the capacitive component in the intermediate circuit.
If the electric energy store were to be connected directly to the capacitive component without pre-charging, an extremely high current would flow instantaneously until the capacitive component were charged, since an energy store, for example, a battery, has a low internal resistance and the capacitive component acting as an intermediate-circuit capacitor has a high electrical capacitance. This would result in extreme aging and early failure of these components. Therefore, it is necessary to pre-charge the capacitive component by applying moderate current.
In order to be able to disconnect the energy storage device or its energy storage modules in a corresponding vehicle from the vehicle's on-board electrical system, two switching devices designed as power switches, usually contactors, are typically provided, each being arranged in one of the main current paths. However, when switching on the two contactors, a considerable current would flow into the capacitive component forming the intermediate circuit capacitor. Therefore, a pre-charging current path including a so-called pre-charging contactor and including a protective resistor is provided, which is connected in parallel with the contactor in the first main current path.
For pre-charging, first, the pre-charging contactor and the main contactor attached to the other terminal of the electric energy storage device, i.e., in the second main current path, are switched on. As a result, the capacitive component acting as an intermediate-circuit capacitor is initially charged with a limited current. As soon as the voltage across the capacitive component is sufficiently large, the main contactor is switched on.
Disadvantages of the described method include the relatively high cost, the relatively large installation space required, and the weight of the pre-charging contactor and the component providing the pre-charging resistance.
The electric energy storage device according to the present disclosure having the features mentioned in claim 1 and the method according to the present disclosure having the features mentioned in claim 9 offer the advantage that the charging of a capacitive component may be carried out in a simple, gentle, and space-saving manner.
In the electric energy storage device, it is provided that the at least one energy storage module includes a controllable multiple-voltage level converter for an optional connection of one or several of its storage units in the current path for the incremental adjustment of a module voltage, as a function of a control signal of a control and/or regulating device of the electric energy storage device. With the aid of the multiple-voltage level converter(s), the voltage at the terminals of the electric energy storage device may be increased in small voltage increments. These voltage increments are smaller than the voltage of a single storage unit of the at least one energy storage module which includes the controllable multiple-voltage level converter. Such an energy storage device is particularly suitable for pre-charging a capacitive component connected to the terminals of the energy storage device, in particular an intermediate-circuit capacitor of an intermediate circuit downstream from the energy storage device. Due to the small increments when increasing the voltage, high current peaks do not result in the capacitive component; therefore, this component does not age so rapidly.
The smallest storage units of the electric energy storage modules are the storage cells. The electric energy storage device is in particular a battery device including battery modules which are connected to form at least one series circuit. At least one of these battery modules includes multiple battery units, preferably battery cells, and a controllable multiple-voltage level converter (multi-level converter). Such a battery device may, for example, be designed as a traction battery device for electric or hybrid vehicles.
According to one preferred embodiment of the present disclosure, the maximum module voltage lies is twice as high or more than twice as high as the voltage of the individual storage units of the at least one energy storage module including the controllable multiple-voltage level converter.
In particular, the maximum module voltage is in the extra-low-voltage range (also colloquially referred to as the low-voltage range or weak-current range). Extra-low voltage (ELV) is a voltage whose upper voltage limit does not exceed the limit value of the voltage range I according to IEC 60449. The limit value for DC voltage is 120 V. This value corresponds to the limit for the permanently permissible contact voltage for adult humans and normal application cases, which is not deemed to be life-threatening. Safety extra-low voltage (SELV) is a voltage which provides particular protection from electric shock due to its low level. If the nominal DC voltage is less than 60 V, protection from direct contact is not necessary. Particularly preferably, the maximum module voltage is in the safety extra-low voltage range, i.e. at most 60 V.
According to one advantageous specific embodiment of the present disclosure, the maximum module voltage is in the range 50 V≤X<60 V. The electric energy storage device may be made up of relatively few energy storage modules, it being possible to handle the modules without special knowledge or special equipment with respect to voltage, for example, when exchanging a module in a repair shop. On the other hand, it is ensured via the multiple-voltage level converter(s) and the control and/or regulating device that the voltage may be increased in substantially smaller voltage increments.
According to one advantageous embodiment of the present disclosure, all energy storage modules of the device include multiple storage units and a multiple-voltage level converter. According to one alternative embodiment of the present disclosure, at least one of the energy storage modules does not include a controllable multiple-voltage level converter. This at least one energy storage module is configured by means of an associated switching device to optionally accommodate the at least one storage unit of this module in one section of the current path or alternatively to short-circuit this section of the current path.
According to another preferred embodiment of the present disclosure, each of the controllable multiple-voltage level converters includes a circuit arrangement including controllable switches and including diode elements. With the aid of such electric components, it is possible to structure a controllable multiple-voltage level converter in a relatively simple manner. The controllable switches are preferably designed as power semiconductor components, in particular as power transistors such as power MOSFETs (metal-oxide semiconductor field-effect transistors). The diode elements are semiconductor components which allow current to pass in only one direction and which block the current, i.e., have high resistance, in the other direction.
Advantageously, the energy storage device includes a parallel circuit made up of multiple current paths connecting the two terminals electrically. Energy storage modules are situated in each of the current paths. A sufficient output current is available due to the parallel circuit.
According to yet another preferred embodiment of the present disclosure, the energy storage device includes a capacitive component acting as an intermediate-circuit capacitor, which is arranged in a current path between the terminals. Advantageously, it is thus provided that the energy storage device includes a converter device connected to the two terminals in parallel with the current path including the capacitive component. An electric machine may then be connected to this converter device.
In the method according to the present disclosure for increasing the voltage at the terminals of an electric energy storage device, in particular an aforementioned electric energy storage device, it is provided that the electric energy storage device includes at least one current path electrically connecting the two terminals, in which multiple energy storage modules are connected to form a series connection of these modules, wherein at least one of the energy storage modules, in particular each of these energy storage modules, includes multiple storage units, preferably storage cells, wherein the at least one energy storage module includes a controllable multiple-voltage level converter for an optional connection of one or several of its storage units in the current path for incrementally increasing a module voltage as a function of a control signal, and the voltage at the terminals is increased by controlling the multiple-voltage level converters in an increment which is at least on average smaller than the maximum module voltage of the at least one energy storage module including the multiple-voltage level converter. In this context, the increment may be understood to mean the height of the voltage step. The method is in particular a method for pre-charging a capacitive component connected to the terminals.